THERMAL CHARACTERISTICS OF ABSORPTION-DRIVEN AND STEAM-HEATED LONG TUBE VERTICAL FALLING FILM EVAPORATORS
Falling film steam-heated evaporators are used in the food industry but their thermal characteristics are not well known. Absorption-driven falling film evaporators represent a new concept in evaporation. In these evaporators CaCl(,2), LiCl or LiBr brine flows down the outside surface of falling film evaporator tubes while the liquid food flows down the inside surface of the same tubes. If the brine concentration is such that its boiling point is higher than the boiling point of the food there will be a temperature difference between the brine and the food which will induce heat transfer and evaporation of water from the food. The vapor generated from the evaporation of the water is transferred through a vapor return line and absorbed by the brine.
In this work a long tube falling film evaporator was built and operated as a steam-heated evaporator and as an absorption-driven evaporator. The thermal characteristics of both types were studied. It was found that: (a) in both modes of operation the overall heat transfer coefficient, U(,o), is a weak function of the food side Reynolds number, Re(,f), for 1000 < Re(,f) < 4000. (b) U(,o) decreases drastically when the temperature difference between the heating medium and the evaporating medium increases. (c) U(,o) increases with brine Reynolds number, Re(,b), for Re(,b) < 600, but it does not change for 600 < Re(,b) < 1300 and (d) U(,o) does not change with evaporation temperature in absorption-driven evaporation but it decreases as evaporation temperature increases in steam-heated evaporation.
The brine temperature profile is affected by such factors as inlet brine temperature, Re(,b) and amount of non-condensables present in the system.